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Publications of the Astronomical Society of the Pacific 100:1405-1422, September 1988

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Publications of the Astronomical Society of the Pacific 100:1405-1422, September 1988 Publications of the Astronomical Society of the Pacific 100:1405-1422, September 1988 STELLAR PHOTOMETRY IN THE PHOENIX DWARF GALAXY* SERGIO ORTOLANI Osservatorio Astronómico di Padova, Vicolo dellOsservatorio 5, 35100 Padova, Italy AND RAFFAELE G. GRATTON Osservatorio Astronómico di Roma, Via del Parco Mellini 84, 00136 Roma, Italy Received 14 May 1988 ABSTRACT CCD photometry in Β and V, obtained at the ESO 2.2-m telescope, for a field in the Phoenix dwarf galaxy, is presented. Our data indicate that the galaxy is dominated by an old metal-poor population, similar to that of the galactic globular clusters M 3 and M 92. The distance modulus is somewhat uncertain ((m — M)v = 23.5 ± 0.5), but there are indications that the Phoenix dwarf galaxy belongs to the Local Group. A small amount (~ 104 SKq) 0fstars 0fa young population {t ~ 107 yrs) is also present. We suggest the Phoenix dwarf galaxy is a dwarf spheroidal galaxy in which a second recent burst of star formation has occurred. Key words: galaxies: individual-photometry-galaxies: Local Group I. Introduction population of galaxies belonging to the Local Group, it An object of low surface brightness, apparently com- would be interesting to have some more insight into the posed of stellar images, was discovered by Schuster and nature of the Phoenix dwarf galaxy. West (1976) from an inspection of a ß plate for the ESO During an observing run in October 1984, we acquired (ß) survey, in Phoenix (a(1950) = 1^9^0, δ(1950) = a series of CCD frames through (7, B, V, and Ha filters of 44042'; ( = 272°, b = 69°). More extensive observations various regions in the galaxy. While most of the resulting by Canterna and Flower (1977), including photographic frames were of rather poor quality, we were able to have a photometry using the Racine wedge at the CTIO 4-m couple of good deep Β and V frames of the western region telescope, suggested that the object is a dwarf galaxy, of the galaxy, including the association of blue stars de- probably irregular, since some of the brightest stars (con- scribed by Canterna and Flower. A preliminary report centrated in a region on the west side of the object) are about these observations was presented in Gratton, Or- blue. Canterna and Flower estimated a distance modulus tolani, and Richter (1986). In this paper we present of26.33 ± 0.16 mag, by matching the C-M diagram they panoramic photometry of about 700 stars down to V = 24 obtained for the brightest stars (V <21.2) with the C-M within this frame. diagram of the brightest stars in IC 1613 (Sandage 1971; Π. Observations Sandage and Katem 1976). However, this procedure is quite ambiguous since the galaxy might be considerably A field (Fig. 1) in the Phoenix dwarf galaxy was ob- closer to us. This eventuality is also supported by the served both in Β and V in October 1984 with a back-illu- apparent lack of Η II regions. Considering this possibility, minated RCA CCD (ESO No. 2) at the Cassegrain focus of Richter (1984) included the Phoenix dwarf galaxy in a the European Southern Observatory (ESO) 2.2-m tele- catalog of possible Local Group members. Morras and scope at La Silla. The field is centered approximately 1 arc Bajaja (1986) surveyed at a rather coarse resolution the min W of the galaxy center. Bright UBV equatorial stan- Η I emission from a region including the Phoenix dwarf dards from Landolt (1983) were also observed on the same galaxy. They concluded that most of the radiation is due to night for calibration purposes. the neighboring Magellanic Stream, and it is not related The journal of observations is in Table I where filters, to the galaxy. A component possibly related to the galaxy exposure times, date and time of observation, and seeing _1 measured on the frames themselves (FWHM) are given. was detected at V(lsr) = 128 km s . Unfortunately, the -1 optical velocity of the galaxy is unknown. Given the small The scale of the array is approximately 0'/36 pixel . The resulting field is thus about 2 arc min X 3 arc min on the *Based on observations made at the ESO La Silla Observatory. 320 X 512 pixels target. 1405 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 1406 ORTOLANI AND GRATTON Fig. 1-Reproduction of the CCD program field, taken with the ESO RCA/CCD camera at the ESO 2.2-m telescope. The size of the field is 120" EW X 180" NS (320 X 512 pixels). North is down and east is to the right. The frames were reduced for readout bias, flat field, the Garching facilities of the European Southern Obser- and dark current using suitable exposures taken during vatory. The last version of DAOPHOT code (including the same nights, by means of conventional procedures the ALLSTAR routine) was used. The point-spread func- (Mould, Kristian, and Da Costa 1983). tion (PSF) for each frame was defined from the sum of several relatively uncrowded, brighter stars. The usual ΠΙ. Reduction iterative process was followed of subtracting the ALL- Analysis of our CCD frames was done with the STAR-fitted stars from the frame, identiiying additional DAOPHOT code (Stetson 1987) loaded within the ones to be added to the list, and rerunning the fitting MIDAS-system running on the VAX-8600 computer at routine. About 700 stars were detected and measured in © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PHOENIX DWARF GALAXY 1407 TABLE I TABLE II Journal of Observations Photometry Frame Filter Date U.T. Exp. time Seei ng ( sec ) Star σ( V) B-V σ(Β) U 18/10/198^ 03:06 3600 T'S 25 6 18/10/198^ 04:09 2700 1 187.0 4.7 21.875 0.020 0.987 0.045 ^6 V 18/10/1984 04:56 1800 ^6 2 135,6 4.7 23.648 0.073 0.854 0. 124 27 Ha 18/10/1984 05:30 2700 11¼ 3 56.5 5.4 23.552 0.067 1 .091 0. 148 4 238. 1 14.6 21.585 0.017 1 . 155 0.027 5 222. 1 21.6 21.740 0.019 1 .213 0.035 6 258.3 23.3 21.056 0.010 1 .323 0.027 both the colors. 7 17.6 24.0 23.855 0. 100 0.641 0. 127 Zero points of the photometry were derived by measur- 8 129.3 26.4 22.517 0.033 0.761 0.033 ing the magnitude of some uncrowded images in each 9 87.5 39. ^ 21.335 0.018 1 .043 0.024 10 58.8 ^5.1 23.7½ 0.099 0.587 frame by means of HP-IHAP aperture magnitudes (at the 0. 103 11 7^.9 50.8 22.418 0.035 0.935 0.0^6 ESO Garching center) using a 7'/3-square aperture. The 12 220.9 51.6 21.032 0.012 1 .389 0.029 transformation to the standard Johnson system was en- 13 22^.1 58.8 23.252 0.056 1.138 0.1^5 sured by analogous reduction of bright equatorial stan- 14 5.7 61.3 23.779 0.169 0.803 0.131 dards (Landolt 1983). The transformation equations from 15 115.6 65.2 23.896 0.126 0.383 0.097 IHAP to Johnson BV magnitudes are: 16 249.8 66.3 21.999 0.018 0.819 0.025 17 186.0 75.8 21.895 0.016 1 .070 0.0M V = tw + (24.687 ± 0.008) (1) 18 61.6 71 .8 24.725 0.52^ 1.873 0.768 19 222.^ 73.8 22.0^9 1.189 0.039 and 0.025 20 96.7 78.3 22.158 0.028 0.777 0.0^0 {Β-V) = 1.121 (b - v)mA? + (0.436 ± 0.024) . (2) 21 9.0 77.9 23.830 0.077 -0.0^5 0.056 22 23^.3 87.1 23.580 0.082 1 .016 0.209 Equation (1) is valid for a 10-sec V exposure, while equa- 23 113.7 86. 1 21.732 0.016 1 .199 0.023 tion (2) holds for a ratio of 2 between the exposure times in 24 152.2 86. ^ 21.202 0.018 1.264 0.025 b and v. 25 89.6 90.3 23.791 0.100 0.^41 0.093 The color equations (1) and (2) are briefly discussed in 26 177.5 93.2 22.898 0.0^7 0.635 0.070 Gratton and Ortolani (1987) where observations of the 27 291.8 9^.0 23.926 0.143 0.085 0.089 cluster LMC:C0 435-589 in Reticulum, made during the 28 20.3 94.1 23.638 0.111 0.659 0.092 29 99.9 9^.^+ 22.811 0.0^8 0.871 0.070 same observing nights as the present observations of the 30 273.7 9^.4 22.846 0.067 0.82^ 0.085 Phoenix galaxy, are presented. We think that the magni- 31 163.6 96.5 23.773 0.065 0.723 0.128 tude scales of the present paper have errors not larger 32 111.9 99.3 22.513 0.03V 0.699 0.050 than 0.05 mag both in V and (Β — V) in the range 18 < V 33 57.8 101 .8 2^.080 o.
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